x86/mm: Make flush_tlb_mm_range() more predictable

I'm about to rewrite the function almost completely, but first I
want to get a functional change out of the way.  Currently, if
flush_tlb_mm_range() does not flush the local TLB at all, it will
never do individual page flushes on remote CPUs.  This seems to be
an accident, and preserving it will be awkward.  Let's change it
first so that any regressions in the rewrite will be easier to
bisect and so that the rewrite can attempt to change no visible
behavior at all.

The fix is simple: we can simply avoid short-circuiting the
calculation of base_pages_to_flush.

As a side effect, this also eliminates a potential corner case: if
tlb_single_page_flush_ceiling == TLB_FLUSH_ALL, flush_tlb_mm_range()
could have ended up flushing the entire address space one page at a
time.

Signed-off-by: Andy Lutomirski <[email protected]>
Acked-by: Dave Hansen <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Denys Vlasenko <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Josh Poimboeuf <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Nadav Amit <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Link: http://lkml.kernel.org/r/4b29b771d9975aad7154c314534fec235618175a.1492844372.git.luto@kernel.org
Signed-off-by: Ingo Molnar <[email protected]>

arch/x86/mm/tlb.c

@@ -309,6 +309,12 @@ void flush_tlb_mm_range(struct mm_struct *mm, unsigned long start,
 	unsigned long base_pages_to_flush = TLB_FLUSH_ALL;
 
 	preempt_disable();
+
+	if ((end != TLB_FLUSH_ALL) && !(vmflag & VM_HUGETLB))
+		base_pages_to_flush = (end - start) >> PAGE_SHIFT;
+	if (base_pages_to_flush > tlb_single_page_flush_ceiling)
+		base_pages_to_flush = TLB_FLUSH_ALL;
+
 	if (current->active_mm != mm) {
 		/* Synchronize with switch_mm. */
 		smp_mb();
@@ -325,15 +331,11 @@ void flush_tlb_mm_range(struct mm_struct *mm, unsigned long start,
 		goto out;
 	}
 
-	if ((end != TLB_FLUSH_ALL) && !(vmflag & VM_HUGETLB))
-		base_pages_to_flush = (end - start) >> PAGE_SHIFT;
-
 	/*
 	 * Both branches below are implicit full barriers (MOV to CR or
 	 * INVLPG) that synchronize with switch_mm.
 	 */
-	if (base_pages_to_flush > tlb_single_page_flush_ceiling) {
-		base_pages_to_flush = TLB_FLUSH_ALL;
+	if (base_pages_to_flush == TLB_FLUSH_ALL) {
 		count_vm_tlb_event(NR_TLB_LOCAL_FLUSH_ALL);
 		local_flush_tlb();
 	} else {